L. Vincetti

Complex FEM modal solver of optical waveguides with PML boundary conditions

A full-wave modal analysis of two-dimensional, lossy and anisotropic optical waveguides using the finite element method (FEM) is presented. In order to describe the behavior of radiating fields, anisotropic perfectly matched layer boundary conditions are applied for the first time in modal solvers. The approach has been implemented using high order edge elements. The resulting sparse eigenvalue algebraic problem is solved through the Arnoldi method. Application to an antiresonant reflecting optical waveguide is reported.

Leakage properties of photonic crystal fibers

An analysis of the con.nement losses in photonic crystal fibers due to the finite numbers of air holes is performed by means of the finite element method. The high flexibility of the numerical method allows us to consider fibers with regular lattices, like the triangular and the honeycomb ones, and circular holes, but also fibers with more complicated cross sections like the cobweb fiber. Numerical results show that by increasing the number of air hole rings the attenuation constant decreases. This dependence is very strong for triangular and cobweb fibers, whereas it is very weak for the honeycomb one.

Hypocycloid-shaped hollow-core photonic crystal fiber Part I: arc curvature effect on confinement loss.

We report on numerical and experimental studies showing the influence of arc curvature on the confinement loss in hypocycloid-core Kagome hollow-core photonic crystal fiber. The results prove that with such a design the optical performances are strongly driven by the contour negative curvature of the core-cladding interface. They show that the increase in arc curvature results in a strong decrease in both the confinement loss and the optical power overlap between the core mode and the silica core-surround, including a modal content approaching true single-mode guidance. Fibers with enhanced negative curvature were then fabricated with a record loss-level of 17 dB/km at 1064 nm.

Characterization of microstructured optical fibers for wideband dispersion compensation

Microstructured optical fibers (MOFs) with small hole-to-hole spacing and large airholes are designed to compensate the anomalous dispersion and the dispersion slope of single-mode fibers. The geometrical parameters that characterize triangular MOFs are chosen to optimize the fiber length and the compensation over a wide wavelength range. A proper design of the photonic crystal fiber geometry allows us to achieve dispersion values of approximately -1700 ps nm(-1) km(-1) at 1550 nm and to compensate the dispersion of standard fibers within +/- 0.5 ps nm(-1) km(-1) over a 100-nm range. The MOF dispersion properties have been studied by means of a numerical simulator for modal analysis based on the finite-element method.

Waveguiding mechanism in tube lattice fibers

Waveguiding mechanism and modal characteristics of hollow core fibers consisting of a single or a regular arrangement of dielectric tubes are investigated. These fibers have been recently proposed as low loss, broadband THz waveguides. By starting from a description in terms of coupling between air and dielectric modes in a single tube waveguide, a simple and useful model is proposed and numerically validated. It is able to predict dispersion curves, high and low loss spectral regions, and the conditions to ensure the existence of low loss regions. In addition, it allows a better understanding of the role of the geometrical parameters and of the dielectric refractive index. The model is then applied to improve the tradeoff between low loss and effectively single mode propagation, showing that the best results are obtained with a heptagonal arrangement of the tubes.

Amplification properties of Er/sup 3+/-doped photonic crystal fibers

The amplification properties of different photonic crystal fibers have been studied by means of a full vector finite-element modal formulation combined with a population and propagation rate equation solver. A honeycomb as well as a cobweb photonic crystal fiber have been considered. The consequences of the defect dimension and the dopant radius on the field intensity distribution as well as the overlap integrals have been analyzed. Results demonstrate that a proper photonic crystal fiber design can be usefully exploited in order to obtain active fibers with superior characteristics compared to standard step index ones. In particular, photonic crystal fibers open up the possibility of a gain medium with highly flexible geometric, dispersion, and amplifying characteristics.

Hypocycloid-shaped hollow-core photonic crystal fiber Part II: cladding effect on confinement and bend loss.

We report on numerical and experimental studies on the influence of cladding ring-number on the confinement and bend loss in hypocycloid-shaped Kagome hollow core photonic crystal fiber. The results show that beyond the second ring, the ring number has a minor effect on confinement loss whereas the bend loss is strongly reduced with the ring-number increase. Finally, the results show that the increase in the cladding ring-number improves the modal content of the fiber.

Flexible tube lattice fibers for terahertz applications.

In this paper a flexible hollow core waveguide for the terahertz spectral range is demonstrated. Its cladding is composed of a circular arrangement of dielectric tubes surrounded by a heat-shrink jacket that allows the fiber to be flexible. Characterization of straight samples shows that the hollow core allows the absorption caused by the polymethylmethacrylate tubes of the cladding to be reduced by 31 times at 0.375 THz and 272 times at 0.828 THz with respect to the bulk material, achieving losses of 0.3 and 0.16 dB/cm respectively. Bending loss is also experimentally measured and compared to numerical results. For large bending radii bending loss scales as Rb-2, whereas for small bending radii additional resonances between core and cladding appear. The transmission window bandwidth is also shown to shrink as the bending radius is reduced. An analytical model is proposed to predict and quantify both of these bending effects.

Exact evaluation of the Jones matrix of a fiber in the presence of polarization mode dispersion of any order

Describes how to evaluate exactly in a recursive manner the Jones transfer matrix of a fiber for any order of polarization mode dispersion (PMD) and derives for it a second-order accurate analytical approximation. The second-order approximation is then compared against the exact result as well as with other models found in the literature.

Multi-meter fiber-delivery and pulse self-compression of milli-Joule femtosecond laser and fiber-aided laser-micromachining

We report on damage-free fiber-guidance of milli-Joule energy-level and 600-femtosecond laser pulses into hypocycloid core-contour Kagome hollow-core photonic crystal fibers. Up to 10 meter-long fibers were used to successfully deliver Yb-laser pulses in robustly single-mode fashion. Different pulse propagation regimes were demonstrated by simply changing the fiber dispersion and gas. Self-compression to ~50 fs, and intensity-level nearing petawatt/cm(2) were achieved. Finally, free focusing-optics laser-micromachining was also demonstrated on different materials.